HBB and Hemoglobin S Measurement: SCA is defined as a monogenic hemoglobin disorder caused by homozygosis for A-to-T transversion at codon 7 (c.20A > T, p.E7V) in the hemoglobin subunit beta (HBB) gene (den Dunnen and Antonarakis, 2000; Steinberg, 2008).